The kitten command

Bash, 19 bytes

cat "$@"|tr A-Z a-z

The best way to make kittens is to use actual cats.

Example run

$ ./kitten kitten
cat "$@"|tr a-z a-z

Perl, 6 bytes

5 bytes code + 1 byte command line

$_=lc

Example usage:

echo ABCdef | perl -p kitten.pl
abcdef

Confirmation of correct Unicode behaviour:

echo "HelloÉ" | perl -p kitten.pl
helloÉ

Perl, 11 bytes

10 bytes code + 1 byte command line

y/A-Z/a-z/

Example usage:

perl -p entry.pl input1.txt input2.txt
echo "ABCdef" | perl -p entry.pl